Conventionally, resistance change type, capacitance change type, zirconia (ZrO2) solid oxide type humidity sensors, etc. have been known as humidity sensors for detecting a vapor density in measured gas.
There are advantages in which the polymer membrane resistance change type humidity sensors are inexpensive and are easy to from a device. On the other hand, there are disadvantages in which measurement accuracies in low-humidity regions are low, and temperature dependency is large, and elution due to condensation leads to element deteriorations and breakdowns.
Moreover, there are advantages in which the capacitance change type humidity sensors have sufficient linearity, are measurable in whole region of relative humidity, and have small temperature dependency. On the other hand, there is disadvantages in which the capacitance change type humidity sensors have significant effects of waters (tap water etc.) other than a pure water and organic solvents, etc. Moreover, while capacitance in humidity of 0% RH is hundreds of pF, a capacitance change when 1% RH is changed is equal to or less than 1 pF, and therefore periodic calibration is required for exact humidity measurement. While the capacitance change type humidity sensor is an effective device when highly accurate humidity measurement is not required in ordinary office environments, it is unexpected usage in highly accurate humidity measurement, a possible atmosphere of condensation or exposure to gas (for weather observations, or for bathroom), and atmospheres of high temperatures equal to or greater than 100 degrees Celsius (degrees C.). Polymeric materials need to be newly developed to improve durability thereof. While future research and development are expected, novel material developments require many time periods and costs.
There have been commercially available humidity sensors using zirconia solid electrolytes used for humidity measurements in high temperatures. Oxygen sensors using zirconia solid electrolytes have been used for the purpose of improving combustion efficiencies of automobiles, and reduction of NOx, and there is sufficient results for durability as materials. However, since the temperature of zirconia is increased to hundreds of degrees C. when using it, power consumption is as high as 100 W, and handling of high-temperature substance is still more difficult, its mart is limited to apart of industrial uses.
Consequently, in recent years, attention is being given to limiting-current type sensors using zirconia thin film. Such a kind of the limiting-current type oxygen sensor has an advantages of high reliability and sufficient linearity.